THURSDAY, Aug. 12 (HealthDay News) -- Less than four hours of sleep on five consecutive nights may affect the brain in a way similar to that of acute total sleep deprivation, a new study suggests.

The finding, from animal research, adds to growing evidence about the negative effects that lack of sleep has on both the brain and the body, said study leader Dr. Chiara Cirelli, an associate professor of psychiatry at the University of Wisconsin-Madison School of Medicine and Public Health.

She and her colleagues studied the brain waves of rats who were kept awake 20 hours a day over five days. Specifically, they focused on slow-wave activity (SWA), which reveals an individual's need to sleep as well as the intensity of sleep that follows a period of wakefulness.

The rats' SWA measures showed that sleep restriction produced intense recovery sleep (longer and deeper sleep) following each wake cycle. The more effective the researchers were at keeping the rats awake during the 20 hours each day, the larger the rodents' sleep rebound.

"It was an indirect but powerful indication of how sleepy the animals actually were," Cirelli said in a university news release.

The study appears in the current online edition of the Proceedings of the National Academy of Sciences.

Cirelli noted that many people are sleep-restricted, either by necessity or by choice.

"Instead of going to bed when they are tired, like they should, people watch TV and want to have an active social life," Curelli said. "People count on catching up on their sleep on the weekends, but it may not be enough."

"Even relatively mild sleep restriction for several nights can affect an individual's ability to perform cognitive tasks," she concluded. "For instance, recent studies in humans have shown that five days with only four hours of sleep/night result in cumulative deficits in vigilance and cognition, and these deficits do not fully recover after one night of sleep, even if 10 hours in bed are allowed. Sleep restriction can also increase resistance to insulin, leading to a risk of diabetes."